As a method of manufacturing seamless steel tubes under hot working conditions, the Mannesmann tube-making process is widely employed. In this tube-making process, a round billet heated to a high temperature is fed as a material to be rolled into a piercing mill (a so-called "piercer"), which pierces the axial center portion of the round billet to yield a hollow shell. The thus-obtained hollow shell is fed directly, or, as needed, after undergoing an expansion or wall-thinning process in an elongator having the same structure as that of the piercing mill, into a subsequent elongating mill such as a plug mill, a mandrel mill, or the like so as to be elongated. Subsequently, the thus-elongated tube undergoes a finishing process performed by a stretch reducer for shape correction, a reeler for polishing, and a sizer for sizing, thereby being formed into a seamless steel tube product.
FIG. 1 shows the material being pierced and an arrangement of piercing rolls and disk rolls in a piercing mill. Piercing rolls 2, 2 are axisymmetrically arranged so as to form a predetermined feed angle with respect to the pass line X-X along which a round billet 1 serving as the material is rolled. On the other hand, a disk roll 3 is close to and perpendicular to the piercer rolls 2, and opposedly arranged with a material 1 to be rolled to piercing. In the piercing mill with the piercing rolls 2, 2 arranged as above, when the round billet 1 is fed along the pass line X-X in the direction indicated by the illustrated white arrow, the round billet 1 is caught between the piercing rolls 2 and is transferred along the pass line X-X while being rotated, during which the round billet 1 is pierced at its axial center portion by an unillustrated plug so as to become a hollow shell.
A piercing mill which adopts a roll arrangement such that a pair of piercer rolls are arranged one above the other and opposite each other with respect to a pass line is usually called a "vertical piercing mill," and a piercing mill wherein a pair of piercer rolls are arranged one beside the other and opposite each other with respect to a pass line is usually called a "orizontal piercing mill." Recently, the vertical piercing mill has been widely adopted as a seamless steel tube manufacturing apparatus, because of an advantage thereof that changeover work can be performed in a simple manner, as will be described later.
As previously mentioned, since the piercer rolls mounted on the piercer are axisymmetrically arranged so that they are at a predetermined angle and roll-shaped with respect to the material, when a dimension of the material is changed due to the step exchange, it is necessary to exchange the piercer rolls. Further, in the piercer, since the material is processed while pressing down the piercer rolls against the material maintained at a high temperature, the processing surfaces of the rolls are deteriorated as the rolling time passes. To this end also, the piercer rolls are periodically exchanged. Generally, the piercer rolls are received in a cylindrical cradle so that they are supported at a predetermined feed angle and, if necessary cross angle, in which state they are mounted on the rolling mill body. Therefore, the exchange of the piercer rolls is carried out in every cradle.
In exchanging the piercer rolls of the foregoing horizontal piercing mill, a mill housing of the piercing mill must be opened and the cradles must be suspended successively by means of suspension means such as an overhead traveling crane, or the like. In piercer roll exchange work employing suspension means, such as an overhead traveling crane, much time is required for exchanging a pair of piercer rolls and hence a decrease in availability of the piercing mill is unavoidable.
Even the "vertical piercer", if it is the type in which cradles are exchanged in order one-by-one, the roll exchanging work by way of the hanging device, a problem similar to that described above occurs. Therefore, as an improvement of the piercer roll exchange in the vertical piercer, the present inventors have proposed a piercer that can exchange rolls without using the hanging device, for example, in Japanese Patent Application Laid-open (kokai) No. 5-200412. That is, this patent discloses a system in which a swing door of a mill housing for covering the rolling mill body can be turned sideways of a pass line to open the mill housing, and rolls are drawn out in a horizontal lateral direction of a pass line. However, the roll exchange system proposed therein is intended mainly for the exchange of disk rolls, and the above patent application discloses no specific device or construction for exchange of piercer rolls. Therefore, in order to automate the piercer roll exchange work in a piercing mill, the conventional piercer roll exchanging methods must be reviewed, an optimum method must be determined, and a piercer roll exchanging apparatus must be developed on the basis thereof